Factors contributing to ice nucleation and sequential freezing of leaves in wheat.

MAIN CONCLUSION: Anatomical, metabolic and microbial factors were identified that contribute to sequential freezing in wheat leaves and likely contribute to supercooling in the youngest leaves and potentially meristematic regions. Infrared thermography (IR) has been used to observe wheat leaves freezing independently and in an age-related sequence with older leaves freezing first. To determine mechanisms that might explain this sequence of freezing several analytical approaches were used: (1) The size of xylem vessels, in proximity to where freezing initiated, was measured to see if capillary freezing point depression explained sequential freezing. The sequence of freezing in the four youngest leaves was correlated, with the largest vessels freezing first. (2) Carbohydrate and amino acids were analyzed to determine if solute concentrations as well as interactions with membranes explained the freezing sequence. Sucrose was highly correlated to the freezing sequence for all leaves suggesting a prominent role for this sugar as compared to other simple sugars and fructans. Among individual free amino acids proline and serine were correlated to the freezing sequence, with younger leaves having the highest concentrations. (3) Microflora within and on leaf surfaces were determined to measure potential freezing initiation. Levels of bacteria and fungi were correlated to the freezing sequence for all leaves, and species or genera associated with high ice nucleation activity were absent in younger leaves. Moisture content and transcript expression of ice binding proteins were also measured. As expected, our results show that no single mechanism explains the freezing sequence observed via infrared analyses. While these multiple mechanisms are operative at different levels according to the leaf age, they seem to converge when it comes to the protection of vital meristematic tissues. This provides potential phenotypic characters that could be used by breeders to develop more winter-hardy genotypes.

Spoilage of hydrogel contact lenses by lipid deposits. Tear-film potassium depression, fat, protein, and alcohol consumption.

"Jelly-bump" deposits, which are composed mainly of lipids, on high water content, extended-wear hydrogel lenses, are a clinically significant problem of unknown etiology. The authors explored the effect of nutrition on the frequency of deposition of these jelly bumps. Forty-three patients were included in this masked case-control study from five contact lens practices in North America. Thirty patients were lipid-depositor cases and 13 were controls. Tear specimens were drawn and analyzed for potassium by atomic-absorption spectrophotometry. Detailed dietary histories were conducted and diet nutrient composition was computed. Patients with diminished tear-potassium concentrations have an increased risk of jelly-bump deposition. Elevated intake of protein, alcohol, and cholesterol also appeared to be correlated significantly with increased jelly-bump deposition. The authors found that diabetics and the use of diuretics, anticholinergics, sympathomimetics, were associated with tear-film potassium depletion and lipid deposit formation.